Resynchronization of follicular wave using long-acting injectable progesterone or estradiol benzoate at 14 days post-TAI in Bos taurus x Bos indicus beef heifers

Abstract We compared the pregnancy rates of beef heifers resynchronized 14 days after first TAI by a P4 intravaginal device associated with either long-acting injectable progesterone (iP4) or estradiol benzoate (EB). Braford and Brangus heifers were submitted to a TAI (D0). On D14, all animals received a P4 intravaginal device and were randomly divided into two groups to receive: EB (1 mg; n = 339); or iP4 (75 mg; n = 338). On D22, P4 devices were removed, and non-pregnant (NP) heifers were identified by assessing morphological luteolysis with Doppler ultrasonography. The NP heifers had the dominant follicle diameter measured and were submitted to a second TAI on D24. The dominant follicle diameter (mm) on D22 in NP heifers did not differ (P > 0.05) between EB (9.77 ± 0.25) and iP4 (9.92 ± 0.22) groups. No difference was observed between EB and iP4 groups for pregnancy rate on D22 (56.3% vs. 60.1%, respectively), and D40 post-first TAI (49.6% vs. 53.3%, respectively). The rate of potential pregnancy losses from D22 to D40 did not differ between EB (12%, 23/191) and iP4 (11.3%, 23/203) groups. The resynchronization pregnancy rate in iP4 group (31.8%, 43/135) was lesser (P